(1) Field of the Invention
This invention relates to a method of repairing mask defects and more particularly to a method of repairing mask defects by repairing the latent image in a layer of exposed resist rather than direct mask repair.
(2) Description of the Invention
In the manufacture of semiconductor integrated circuits masks are often used to form lithographic images in a layer of resist. Defect free masks are important in order to obtain high quality resist images. Since masks are often not perfect, mask repair is an important part of mask fabrication.
U.S. Pat. No. 5,935,737 to Yan describes a method of fabricating extreme ultraviolet, EUV, masks which eliminates damage arising from repairing defects.
U.S. Pat. No. 5,607,776 to Mueller et al. describes a method of in-situ cleaning a Ti target in a Ti+TiN anti-reflective coating process.
U.S. Pat. No. 5,981,110 to George. et al. describes a method for repairing a defect in an opaque layer of a photomask. The method comprises applying a photoresist layer over the opaque layer, removing the photoresist over the defect to reveal the defect, and then repairing the defect using a wet etch.
U.S. Pat. No. 5,795,685 to Liebmann et al. describes a method and apparatus for correcting defects in a phase shifting mask.
U.S. Pat. No. 4,727,234 to Oprysko et al. describes a LASER based apparatus for repairing both clear and opaque defects in a photomask having a metal film pattern on a glass plate.
U.S. Pat. No. 4,548,883 to Wagner describes a method of mask repair using ion beam removal of material at the location of a mask defect.
Masks are used in semiconductor integrated circuit manufacture to expose pattern images in a layer of resist. The exposed resist is then developed to form a resist pattern which serves as a mask to transfer the pattern image to the integrated circuit wafer. It is critically important that the resist pattern thus formed is accurate and free of defects. Any mask defects will be transferred to the resist pattern and mask defects must be carefully controlled.
For binary intensity masks, mask defects can be repaired to prevent transfer of the defects to a layer of resist. FIG. 1 shows a top view and FIG. 2 a cross section view of a binary intensity mask having defects in the mask. The defects can take the form of pinholes 14 in an opaque region 12 of the mask or unwanted opaque material 16 on the transparent mask substrate 10. Conventionally, as shown in FIG. 3, these mask defects can be repaired by removing the unwanted opaque material and filling the pinholes with opaque material 18. These mask repairs can be readily accomplished with conventional binary intensity masks.
In the case of phase shifting masks, PSM, or extreme ultraviolet, EUV, masks, however, these mask repairs are difficult, if not impossible to accomplish. FIGS. 4 and 5 illustrate the problem of mask repair for phase shifting masks. FIG. 4 shows a top view and FIG. 5 a cross section view, taken along line 5-5xe2x80x2 of FIG. 4, of a phase shifting mask having an opaque region 22 with a pinhole defect 24 and defects comprising unwanted opaque material 28 on the mask substrate 20 and a defect 26 in the mask substrate causing a phase defect. As in the case of the binary intensity mask the pinhole defect 24 in the opaque material 22 can be repaired but repair of the other defects is much more difficult. It is difficult to fill the defect 26 in the mask substrate with material having the proper thickness and index of refraction to correct the phase defect. Methods used to remove unwanted opaque material 28 may damage the mask substrate 20 and result in additional phase defects.
FIGS. 10 and 11 illustrate the problem of mask repair for extreme ultraviolet, EUV, masks. FIG. 10 shows a top view and FIG. 11 a cross section view, taken along line 11-11xe2x80x2 of FIG. 10 of an EUV mask. As shown in FIG. 11 the EUV mask is fabricated by forming reflective layers 46, 48, and 50 on a mask substrate. Pattern elements 42 are formed from opaque absorptive material. One type of defect encountered are pinholes 44 in the pattern elements, see FIGS. 10 and 11. Other defects 56 and 58, see FIG. 10, are caused by foreign material 52 and 54, see FIG. 11, between the layers of reflective material 46, 48, and 50. The pinhole defects 44 in the opaque pattern elements 42 are readily repaired but the defects caused by foreign material between reflective layers is very difficult to repair. The defects caused by foreign material between reflective layers will cause underexposed regions of resist.
It is a principle objective of this invention to provide a method of overcoming mask defects to produce a defect free resist image.
This objective is accomplished by direct repair of the resist image. The pinhole type of defects in opaque pattern elements, which would cause overexposed regions of resist and can readily be repaired on the mask, are first repaired directly on the mask before the mask is used in the exposure of a layer of resist. The remaining defects on the mask are left as they are and not repaired. The layer of resist is then exposed using the partially repaired mask. The remaining mask defects will cause unexposed latent images in the layer of resist. These unexposed regions of the resist are then exposed using supplemental radiation thereby correcting the exposure of the layer of resist. The layer of resist is then developed to form a defect free resist image.